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<title>Reverse Iterator</title>
<meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
<meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
<meta name="date" content="2006-09-11" />
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<div class="document" id="reverse-iterator">
<h1 class="title">Reverse Iterator</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek&#64;osl.iu.edu">jsiek&#64;osl.iu.edu</a>, <a class="last reference external" href="mailto:witt&#64;ive.uni-hannover.de">witt&#64;ive.uni-hannover.de</a></td></tr>
<tr><th class="docinfo-name">Organization:</th>
<td><a class="first reference external" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference external" href="http://www.osl.iu.edu">Open Systems
Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport
Railway Operation and Construction</a></td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>2006-09-11</td></tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr>
</tbody>
</table>
<!-- Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<tr class="field"><th class="field-name">abstract:</th><td class="field-body"><!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
The reverse iterator adaptor iterates through the adapted iterator
range in the opposite direction.</td>
</tr>
</tbody>
</table>
<div class="contents topic" id="table-of-contents">
<p class="topic-title first">Table of Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#reverse-iterator-synopsis" id="id2"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> synopsis</a></li>
<li><a class="reference internal" href="#reverse-iterator-requirements" id="id3"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> requirements</a></li>
<li><a class="reference internal" href="#reverse-iterator-models" id="id4"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> models</a></li>
<li><a class="reference internal" href="#reverse-iterator-operations" id="id5"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> operations</a></li>
<li><a class="reference internal" href="#example" id="id6">Example</a></li>
</ul>
</div>
<div class="section" id="reverse-iterator-synopsis">
<h1><a class="toc-backref" href="#id2"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> synopsis</a></h1>
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<pre class="literal-block">
template &lt;class Iterator&gt;
class reverse_iterator
{
public:
  typedef iterator_traits&lt;Iterator&gt;::value_type value_type;
  typedef iterator_traits&lt;Iterator&gt;::reference reference;
  typedef iterator_traits&lt;Iterator&gt;::pointer pointer;
  typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
  typedef /* see below */ iterator_category;

  reverse_iterator() {}
  explicit reverse_iterator(Iterator x) ;

  template&lt;class OtherIterator&gt;
  reverse_iterator(
      reverse_iterator&lt;OtherIterator&gt; const&amp; r
    , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
  );
  Iterator const&amp; base() const;
  reference operator*() const;
  reverse_iterator&amp; operator++();
  reverse_iterator&amp; operator--();
private:
  Iterator m_iterator; // exposition
};
</pre>
<p>If <tt class="docutils literal"><span class="pre">Iterator</span></tt> models Random Access Traversal Iterator and Readable
Lvalue Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
<tt class="docutils literal"><span class="pre">random_access_iterator_tag</span></tt>. Otherwise, if
<tt class="docutils literal"><span class="pre">Iterator</span></tt> models Bidirectional Traversal Iterator and Readable
Lvalue Iterator, then <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is convertible to
<tt class="docutils literal"><span class="pre">bidirectional_iterator_tag</span></tt>. Otherwise, <tt class="docutils literal"><span class="pre">iterator_category</span></tt> is
convertible to <tt class="docutils literal"><span class="pre">input_iterator_tag</span></tt>.</p>
</div>
<div class="section" id="reverse-iterator-requirements">
<h1><a class="toc-backref" href="#id3"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> requirements</a></h1>
<p><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be a model of Bidirectional Traversal Iterator.  The
type <tt class="docutils literal"><span class="pre">iterator_traits&lt;Iterator&gt;::reference</span></tt> must be the type of
<tt class="docutils literal"><span class="pre">*i</span></tt>, where <tt class="docutils literal"><span class="pre">i</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</p>
</div>
<div class="section" id="reverse-iterator-models">
<h1><a class="toc-backref" href="#id4"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> models</a></h1>
<p>A specialization of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> models the same iterator
traversal and iterator access concepts modeled by its <tt class="docutils literal"><span class="pre">Iterator</span></tt>
argument.  In addition, it may model old iterator concepts
specified in the following table:</p>
<table border="1" class="docutils">
<colgroup>
<col width="53%" />
<col width="47%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">If <tt class="docutils literal"><span class="pre">I</span></tt> models</th>
<th class="head">then <tt class="docutils literal"><span class="pre">reverse_iterator&lt;I&gt;</span></tt> models</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>Readable Lvalue Iterator,
Bidirectional Traversal Iterator</td>
<td>Bidirectional Iterator</td>
</tr>
<tr><td>Writable Lvalue Iterator,
Bidirectional Traversal Iterator</td>
<td>Mutable Bidirectional Iterator</td>
</tr>
<tr><td>Readable Lvalue Iterator,
Random Access Traversal Iterator</td>
<td>Random Access Iterator</td>
</tr>
<tr><td>Writable Lvalue Iterator,
Random Access Traversal Iterator</td>
<td>Mutable Random Access Iterator</td>
</tr>
</tbody>
</table>
<p><tt class="docutils literal"><span class="pre">reverse_iterator&lt;X&gt;</span></tt> is interoperable with
<tt class="docutils literal"><span class="pre">reverse_iterator&lt;Y&gt;</span></tt> if and only if <tt class="docutils literal"><span class="pre">X</span></tt> is interoperable with
<tt class="docutils literal"><span class="pre">Y</span></tt>.</p>
</div>
<div class="section" id="reverse-iterator-operations">
<h1><a class="toc-backref" href="#id5"><tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> operations</a></h1>
<p>In addition to the operations required by the concepts modeled by
<tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>, <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> provides the following
operations.</p>
<p><tt class="docutils literal"><span class="pre">reverse_iterator();</span></tt></p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
</tr>
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> with <tt class="docutils literal"><span class="pre">m_iterator</span></tt>
default constructed.</td>
</tr>
</tbody>
</table>
<p><tt class="docutils literal"><span class="pre">explicit</span> <span class="pre">reverse_iterator(Iterator</span> <span class="pre">x);</span></tt></p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> with
<tt class="docutils literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template&lt;class OtherIterator&gt;
reverse_iterator(
    reverse_iterator&lt;OtherIterator&gt; const&amp; r
  , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
);
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="docutils literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="docutils literal"><span class="pre">Iterator</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs instance of <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt> whose
<tt class="docutils literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="docutils literal"><span class="pre">y.base()</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="docutils literal"><span class="pre">Iterator</span> <span class="pre">const&amp;</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">m_iterator</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="docutils field-list" frame="void" rules="none">
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<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"></td>
</tr>
</tbody>
</table>
<pre class="literal-block">
Iterator tmp = m_iterator;
return *--tmp;
</pre>
<p><tt class="docutils literal"><span class="pre">reverse_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">--m_iterator</span></tt></td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="docutils literal"><span class="pre">reverse_iterator&amp;</span> <span class="pre">operator--();</span></tt></p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="docutils literal"><span class="pre">++m_iterator</span></tt></td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<pre class="literal-block">
template &lt;class BidirectionalIterator&gt;
reverse_iterator&lt;BidirectionalIterator&gt;n
make_reverse_iterator(BidirectionalIterator x);
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="docutils literal"><span class="pre">reverse_iterator&lt;BidirectionalIterator&gt;</span></tt>
with a <tt class="docutils literal"><span class="pre">current</span></tt> constructed from <tt class="docutils literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
</div>
<div class="section" id="example">
<h1><a class="toc-backref" href="#id6">Example</a></h1>
<p>The following example prints an array of characters in reverse order
using <tt class="docutils literal"><span class="pre">reverse_iterator</span></tt>.</p>
<pre class="literal-block">
char letters_[] = &quot;hello world!&quot;;
const int N = sizeof(letters_)/sizeof(char) - 1;
typedef char* base_iterator;
base_iterator letters(letters_);
std::cout &lt;&lt; &quot;original sequence of letters:\t\t\t&quot; &lt;&lt; letters_ &lt;&lt; std::endl;

boost::reverse_iterator&lt;base_iterator&gt;
  reverse_letters_first(letters + N),
  reverse_letters_last(letters);

std::cout &lt;&lt; &quot;sequence in reverse order:\t\t\t&quot;;
std::copy(reverse_letters_first, reverse_letters_last,
          std::ostream_iterator&lt;char&gt;(std::cout));
std::cout &lt;&lt; std::endl;

std::cout &lt;&lt; &quot;sequence in double-reversed (normal) order:\t&quot;;
std::copy(boost::make_reverse_iterator(reverse_letters_last),
          boost::make_reverse_iterator(reverse_letters_first),
          std::ostream_iterator&lt;char&gt;(std::cout));
std::cout &lt;&lt; std::endl;
</pre>
<p>The output is:</p>
<pre class="literal-block">
original sequence of letters:                   hello world!
sequence in reverse order:                      !dlrow olleh
sequence in double-reversed (normal) order:     hello world!
</pre>
<p>The source code for this example can be found <a class="reference external" href="../example/reverse_iterator_example.cpp">here</a>.</p>
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